1. Field of the Invention
This invention relates to a method for manufacturing a friction material and in particular, relates to a method for shortening the curing time of a friction material including a thermosetting resin.
2. Description of the Related Art
Many of the friction materials, such as brake linings or clutch facings, are made into products by themselves; however, some friction materials are made into products such as disc pads by fixing the friction material to metallic plates called a back plate or brake shoes by fixing the friction material to metallic brake shoe bodies.
FIGS. 4A and 4B show a back plate of a conventional friction material product, where FIG. 4A is a plan view, and FIG. 4B is a cross sectional view of FIG. 4A taken along the line B-B. This friction material product is used in a disc pad of a disc brake. In the figure, the back plate 1 is made of a steel plate for automobile structural uses or a steel plate for machine structural uses, and is formed into a set shape by blanking, and at the same time, two binding holes 2, 2 are opened therein.
After the blanking process is performed by the pressing machine, oil on the surface of the back plate 1 is removed at a degreasing process, and the surface thereof is finished by sand blasting and the like, and a thermosetting adhesive is applied to the surface so as to increase a binding force between the back plate and the friction material.
A raw material for manufacturing the friction material is a mixture of a fiber material, a filling material, and a binding material, where organic fibers such as cellulose pulp and aramid and the like, metallic fibers such as chip shaped metallic pieces or steel wool and the like, and inorganic fibers such as rock wool and the like are employed as the fiber material. The filling material is for increasing the volume of the friction material or for adding lubricating characteristics of the friction material, thereby obtaining a stable friction characteristic, and for example, barium sulfate, calcium carbonate, graphite and the like are employed as the filing material. The binding material is for binding the fiber material and the filling material, and thermosetting resins such as phenolic resin, urea resin, benzoxazine resin and the like are employed. Raw material of the friction material is made into a friction material through the following processes.
Preliminarily Forming Process
A raw friction material for the friction material, to which each raw material has been mixed, is measured, and a predetermined amount thereof is supplied to a mold, is then pressed by a pressing machine, and is then formed into a preliminarily formed product. In the preliminarily forming, in principle, the preliminarily formed product is made only by pressing; however, in some cases, heating is performed up to a temperature at which the binding material does not react.
FIGS. 5A and 5B are figures of the preliminarily formed product, and FIG. 5A is a plan view thereof, and FIG. 5B is a front view thereof. The shape of the preliminarily formed product 3 is the same as that of a finished product; however, the density is rough, and the thickness T is roughly double the thickness of the finished product that is pressed and fixed to the back plate 1 and compressed to a specified density. Furthermore, in the preliminarily formed product 3, protrusions 4, 4 corresponding to the binding holes 2 are formed. The lower slope portions of the protrusions 4, 4 are wide, but the diameter d1 of the top is smaller than the diameter D1 of the binding hole 2, for easy insertion into the binding hole 2.
Molding Process
The preliminarily formed product 3 formed thus, is superposed onto the back plate 1 on another pressing machine, and is then pressed and heated so as to be fixed.
FIGS. 6A and 6B are figures showing a disc pad 10, that consists of the friction material 13 and the back plate 1, which are fixed by the above process, wherein FIG. 6A is a plan view, and FIG. 6B is a front view. The friction material 13 is adhered on the back plate 1 and bonded to the back plate 1 at the binding hole 2.
Meanwhile, there is a case where the friction material is formed without the process of making the preliminarily formed product. In this case, a powder adhesive layer raw material is placed on the back plate, and the powder raw material used for the friction material or a granulated/size-enlarged material made of the powder raw materials is laminated on the back plate, and then the pressure is applied by a pressing machine thereon to be heated to mold the friction material while the friction material is fixed to the back plate at the same time.
In the case of a type of the friction material which is not fixed to the back plate, there is a case where the friction material is formed via the preliminarily formed product, and a case where without a process of making the preliminarily formed product, the powder raw material or the granulated/size-enlarged material is supplied directly into the mold, pressed and heated to be molded.
Heat Treatment Process
The friction material, which is fixed to the back plate, is bound to the back plate; however, curing of the thermosetting resin included in the friction material is not completed. In the same manner, as in the case of the friction material alone, curing of the thermosetting resin is not completed. Therefore, normally, the friction material is placed in a heating furnace at between 200 and 300° C. ambient temperatures for a time of between 150 and 300 minutes and is then completely cured. This process is called a heat treatment process.
Painting Process
The friction material, after heat treatment completion, is sent to a painting process, wherein spray painting of liquid paint or electrostatic powder painting is performed. Electrostatic powder painting is a painting method, wherein electrostatic powder paint is applied via sprayed onto either the friction material or the friction material fixed on the back plate, which is heated at 200° C. and baked.
Grinding Process
With regard to the painted friction material, as the surface of the friction material is ground, the friction surface is formed, a slit is made in friction surface, and beveled surfaces (chamfers) are formed on both the sides of friction surface.
Heat-Sear (Scorching) Process
In a heat-sear process, the surface of friction surface is seared by a high temperature around 380° C., to secure the braking effectiveness of the new product.
Inspection Process
Thereafter, it is inspected and shipped. Excluding the heat treatment process, the total time required for all of the above process is 1 to 2 minutes; however, the time for the heat treatment process is several hours, which is extremely longer than the other processes and as such, it is a bottleneck in the friction material manufacturing processes. Therefore, it hinders cost reduction.
As a method for shortening this time, there is proposed Patent Document 1 (Provisional Japanese Patent Publication No. 10-204187). Herein, a method is disclosed, wherein the friction material is pressed by a porous plate and heat treatment is performed in a heating furnace. Gas generated during resin curing is discharged outside via holes in the porous plate. As gas can be discharged easily, it is possible to prevent blisters or cracks from occurring in the friction material and therefore allows performing the heat treatment in a short period of time. However, this method uses conventional temperature, and still requires over 2 hour time.
Further, in Patent Document 2 (Provisional Japanese Patent Publication No. 62-21528), the heat treatment is performed in a non-oxidizing atmosphere at high ambient temperature between 350 and 1000° C. By utilizing high ambient temperature, resin curing is facilitated, and by utilizing the non-oxidizing atmosphere, it is possible to prevent the resin from deteriorating. Nitrogen gas is used for the non-oxidizing atmosphere. However, the volume of the heating furnace is large, and it therefore requires an enormous amount of nitrogen gas, which in turn greatly increases manufacture costs.